The integration of magnetic field sensors in a wide range of applications has created a large market and increased the demand for smaller, more accurate and low cost magnetic field sensors. Standard CMOS technology is one of the most cost efficient approaches to integrate magnetic field sensors into various applications. The ability to sense the magnetic field in all three axes enables an even larger number of applications.
A common magnetic field sensor is a device configured to sense the magnetic field parallel to the chip surface. These sensors are referred to as “vertical Hall Effect sensors”. This type sensor was invented in 1985 and have been studied extensively ever since.
The Hall Effect makes use of the Lorentz Force which deflects moving charges in the presence of a magnetic field which is perpendicular to the current flow through the sensor. The deflection causes a charge separation which causes a Hall electrical field. This electrical field acts on the charge in the opposite direction in regard to the Lorentz Force. Both forces balance each other and create a potential difference perpendicular to the direction of current flow. The potential difference can be measured as a Hall voltage and varies in a linear relationship with the magnetic field for small values. FIG. 1 schematically represents this mechanism for a standard Hall Effect sensor. The vertical Hall Effect sensor works on the same principle. In FIG. 2 the current flow of a standard vertical Hall Effect device and the acting forces under the influence of a magnetic field are shown.
Several different vertical Hall Effect devices have been published in the past but all work based upon the same principle and suffer more or less from the same draw backs. One big disadvantage of these known sensors, in relation to conventional Hall Effect sensors, is the high offset. Offset is the description of an output signal which is present without an input signal. This unwanted signal usually follows a random distribution over different sensor samples and exhibits a nonlinear relationship with both temperature and input voltage.
There have been two main strategies used in an attempt to reduce the offset of conventional and vertical Hall Effect sensors. One strategy is referred to as “sensor combination”. In sensor combination, systematic offsets caused by large scale process tolerances and design flaws of the sensor are addressed by combining four sensors in parallel. The device current in each device flows in a different direction in respect to the crystal structure of the silicon grid. This compensates for stress inducted offset for lateral sensors. Applying this method to vertical sensors reduces the offset caused by design and process variation of the well depth of the active region.
A second approach is referred to as the “spinning current method”. The spinning current method can be applied in addition to the sensor combination and reduces the influence of flicker noise. The spinning current method also reduces the offset caused by symmetric differences of each sensor within the sensor combination.
A sensor architecture where 6 n+ doped contacts are placed on an n doped well which is placed in a p doped substrate is used as example and shown FIG. 3. In FIG. 3, the sensor 10 includes six 6 n+ doped contacts 12, 14, 16, 18, 20 and 22 on an n doped well 24. The n doped well 24 is located within a p doped substrate (not shown in FIG. 3). It can be seen that the outer contacts 12 and 22 are connected to the second contact on the opposite side of the device (contacts 14 and 20, respectively). Four of these sensors 10 are connected as described above as a single sensor assembly. The problems identified herein are not limited to this kind of sensor architecture and will affect all CMOS implemented vertical Hall Effect sensors which are designed with the same strategy.
The approaches described above work very well for lateral sensors but fail under certain condition for vertical Hall Effect sensors. Several measurement examples are given in FIG. 4 where both techniques were applied. In FIG. 4 five identical sensor combinations were measured for offset errors. Each of these sensor combinations was on a different die. The maximum offset of one of the sensors was 150 uV/V which is equal to a magnetic field of about 5 mT at a common sensitivity of 30 mV/VT at 25° C.
FIG. 5 shows an exemplary configuration of four sensors like the sensor 10. For simplicity the short circuits of the outer contacts to the inner contact as shown in FIG. 3 of each single sensor are not displayed, but taken in to account. In FIG. 5, an axis of symmetry divides the sensors into sides Y.1 and Y.2. The two methods described above work well as long as there is no asymmetric sensor in this combination, consequently, Y.1=Y.2.
FIG. 6 shows a FEM simulation of a perfect scenario where all four sensors are identical and symmetrical. FIG. 6 shows that the device combination in this case cancels the offset errors perfectly since each single phase of the spinning current shows less than 400 pV/V.
In instances where all four sensors are symmetrical but different from each other, the device combination stops working and each single phase of the spinning current method shows offsets. However, the average of these phases cancels the offset perfectly and thus the spinning current method works for this case.
The spinning current method stops working, however, in configurations with four sensors which are alike but not symmetrical. In one simulated case the asymmetry was generated by a doping gradient in current direction as shown in FIG. 7. In this scenario all four phases show the same offset and the mean of the four phases, which represents the spinning current output, is shown in FIG. 8.
A second source of offset is due to self-generated parasitic magnetic fields. If a single sensor or a sensor combination is in perfect condition the input resistance in each phase will be equal. In this case the amount of current which goes towards the device in phase one is the same as the amount in phase three but in the opposite direction. The same is true for phases two and four. Each of these currents generates a magnetic field but since the amounts of two currents are always equal the mean magnetic field after four phases is 0 T. This scenario is depicted in FIG. 9.
If for some reason the input resistance is not equal in phases one/three and two/four there will be a residual magnetic field after averaging the different phases of the spinning current method. FIG. 10 shows this scenario. This mismatch in current will lead to a residual offset of the sensor system.
What is needed therefore is a system and method which provides offset reduction for vertical Hall Effect sensors if a gradient or any other disturbance along the current flow direction within the one or more sensors exists.